Frequency transformer including safety circuit



April 12, 1966 w. sTROHMElr-:R ETAL 3,246,227

FREQUENCY TRANSFORMER INCLUDING SAFETY CIRCUIT Filed Dec. l0, 1962 2 Sheets-Sheet 1 F ig. l

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FREQUENCY TRANSFORMER INCLUDING SAFETY CIRCUIT 2 Sheets-Sheet 2 Filed Deo. l0, 1962 [N VEN TOR hmm@ Sflnfnnae/z, II//O Gon/:mes 55m/awo Je QUIROS l BY h/EXNEB //MM/N wml 10 ma A TT OR N E Y United States Patent O 3,2%,227 FREQUENCY TRANSFORMER INCLUDHNG SAFETY CRCUIT Walter Strohmeier, Riehen, Basel-Stadt, Switzerland, Julio Gonsales Bernaldo de Quiros, Madrid, Spain, and Werner Ullmann, Ascona, Switzerland, assignors to Agie AG. fur industrielle Elektronik, Lomme-Locarno, Switzerland, a corporation of Switzerland Fiied Dec. 10, 1962, Ser. No. 243,526 Claims priority, application Switzerland, Dec. 13, 1961, 14,452/ 61 Claims. (Cl. 321-4) The present invention relates to an improved static frequency transformer or frequency converter of the type provided with a power rectification stage and a power output stage for converting the direct-current voltage generated by the power rectification stage into an alternating-current Voltage of predetermined frequency. The 'power rectification stage as well as the power output stage of the frequency transformer of the present invention are provided with controlled semi-conductor rectitiers which receive impulses for their ignition from a first and second ignition circuit.

In frequency transformers of the type aforedescribed, the power output stage contains an output transformer having a primary winding provided with a center-tap. The center-tap is electrically coupled with one pole or terminal of the direct-current voltage source. The two terminals of the primary winding are each respectively connected with the second pole or terminal of the directcurrent voltage source or the outlet of the power rectification stage, through the intermediary of a respective controlled rectier. For purposes of commutation the two terminals of the primary winding are further connected with one another through the agency of a commutation capacitor. The alternating-current output load of desired or predetermined frequency is then removed at the secondary ofthe output transformer. The two controlled semi-conductor rectiers, that is rectitiers with a pnpn semi-conductor layer which exhibit a thyratron characteristic, are alternatingly ignited from time to time. In accordance with the ignition of one or both of the controlled rectifiers, the other controlled rectifier is caused to be extinguished by the commutation voltage which appears at the commutation capacitor. The controlled alternation or change of the conductivity of both controlled rectiiiers thereby corresponds to the frequency of the delivered alternating-current voltage.

However, with static frequency transformers of the aforedescribed type there exists the possibility that one or both of the cont-rolled rectifiers cannot be caused to extinguish in the event that a pronounced capacitive load appears at the secondary side of the output transformer. If such case arises, then consequently both controlled semiconductor rectifiers are conductive, which results Within a very short period of time in their destruction.

Accordingly, it is a primary object of the present invention to provide an improved static frequency transformer of the aforedescribed type which prevents destruction or damage of the controlled semi-conductor rectifier elements of the output stage.

A further important object of the present invention is the provision of an improved static frequency transformer of the aforedescribed type in which there is prevented a destruction of the semi-conductor rectifiers, also when the secondary of an output transformer is too strongly loaded.

A further object of the present invention is to provide van improved frequency transformer readily suitable as ice a voltage generator, particularly for electro-erosion tool machines.

The static frequency transformer according to the present invention is characterized in the provision of a safety or security circuit which is constructed as a logical element or circuit, the input of which is electrically coupled with the input and output of the power output stage, whereas the output of said safety circuit is electrically coupled with a first ignition circuit. The logical circuit prevents the transmission of ignition signals to the controlled rectifiers of the power rectification stage then and only then when the voltage at the input of the power output stage exceeds the voltage at the output thereof by a predetermined value.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specic examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

In the drawings:

FIGURE l is a block diagram of a frequency transformer or converter;

FIGURE 2 diagrammatically illustrates the series of ignition impulses necessary for igniting the controlled rectifiers taken with respect to the supplied control voltage; and

FIGURE 3 is a circuit diagram illustrating details of the frequency transformer or converter shown in FIG- URE l.

Referring now to the frequency transformer or converter illustrated in the block diagram of FIGURE l, it will be seen that the input load or energy is applied through a transformer 10 via its primary winding 11. The primary winding can for example be directly connected to a power supply of 220 volts and operating at a frequency of 50 c.p.s. The transformer 10 is provided with three secondary windings 12, 13 and 14. The secondary winding 12 is connected to a full-wave rectifier provided for the power rectification stage A which rectiyfies the supplied or input alternating-current voltage.

voutput stage B where the direct-current voltage is transformed into an alternating-current voltage of predeterminded frequency. The alternating-current voltage then appears at the output terminals 17.

The secondary winding 13 of the transformer 10 feeds an ignition circuit C. As will be more fully explained hereinafter, the ignition circuit C first rectifies the input alternating-current voltage and produces a series of impulses for igniting the two controlled rectiliers 37, 38 of the power rectification stage A. In order to ignite the controlled rectiers 37, 38 of the power rectification stage A, the ignition circuit C supplies a continuous series of impulses via the conductors 19 and 20. The conductor 19 is electrically coupled with the ignition electrodes of the two rectifiers 37, 38 through the agency of resistors 2l and 22.

The desired frequency of the output voltage appearing at the terminals 17 is determined by a controlling or conthe secondary winding 26 is operatively connected to the ignition circuit C. The ignition circuit C is therewith so constructed that, it only then supplies output impulses via the conductors 19 and 20 when an input signal appears at the transformer terminals 23. If a signal fails to appear at the terminals 23 and, thus also not at the secondary winding 26, then the ignition circuit C does not deliver any impulses, so that there also does not appear a direct-current voltage at the conductors 15 and 16. A consequence thereof is that also practically no power is removed at the power supply network to which the primary winding 11 of the transformer 10 is connected when the control alternating-current voltage fails to appear at the terminals 23 of the input transformer 24.

The secondary winding 27 of the input transformer 24 is operatively connected to an ignition circuit ID which provides the ignition impulses for the two controlled rectifiers 50, 51 of the power output stage B. The ignition impulses for one of the two controlled rectifiers of the power output stage B, such as rectifier 50 appear across the connecting terminals 29 and 30, whereas the ignition impulses for the second controlled rectifier 51 Vappear across the conductors 29 and 31.

The frequency transformer or converter of FIGURE 1 additionally contains a safety or security circuit E. This safety circuit E is, on the one hand, electrically coupled with both of the direct-current carrying conductors 15 and 16 at the inputside of the power'output stage B and, on the Aother hand, is connected with the output terminals 17 at the output side thereof. The safety circuit E forms a logical element or circuit which always cuts-olf or prevents the generation of ignition impulses in the ignition circuit C for a predetermined time when a voltage appears at the conductors 15, 16, whereas when no voltage appears at the output terminals 17. On the other hand, the logical element permits generation of ignition impulses when a voltage appears at the output terminals 17,

'as well as then when no voltage appears at the conductors 15, 16 as well as none at the output terminals 17. By

virtue of the safety circuit E there is thus caused to beV ensuredrthat the different controlled rectifiers cannot become damaged, for example, by a short circuit appearing at the output terminals 17.

If, during operation, there appears at the input terminals 23 of the input transformer Z4 a controlling or control alternating-current voltage of arbitrary frequency within a large range, there is first of all brought about a rectification in the power rectification stage A, and furtherthere is produced an alternating-current voltage possessing the supplied frequency at terminals 23, which then appears at the output terminals 17.

In order to more fully explain the function which the 'ignition control circuits C and D fulfill reference is rnade hereinafter to FIGURE 2. In FIGURE 2a there is depicted the controlling or control alternating-current voltage as a function of time which is applied to the terminals 23 ofthe input transformer 24. When the voltage supplied at terminals 23 reaches the ignition circuit C there is released an oscillation so that a regular, that is to say, uninterrupted or continuous series of impulses appear at the conductors 19 and 20, as such is approximately illustrated in FIGURE 2b. The pulse repetition rate or pulse recurrence frequency is itself given through the oscillating system and is largely uncritical.

The series of impulses illustrated in FIGURE 2b now Y reach the two controlled rectiiiers 3-7, 38 of the power rectification stage A via the resistors 21 and 22, whereby it is insignificant that these ignition impulses are maintained during both half-waves, and indeed for the reason that, the ignition impulses cannot then influence the controlled rectifers when the polarity of the voltage applied to the main line path corresponds to the high resistance or blocking direction of the rectiiiers.

In the ignition circuit D two series of impulses are generated, whereby the first series of impulses as shown in FIGURE 2c, appear only for example during the positive half-wave of the controlling or control alternating-current voltage (FIGURE 2a), and the second series of impulses illustrated in FIGURE 2d appear only during the negative half-wave of the controlling or control alternating-current voltage (FIGURE 2a). The voltage of 'FIG- URE 2c appears for example between the conductors 29 and 30, and the control alternating-current voltage of FIG- URE =2d between the conductors 29 and 31. The pulse repetition rate of the series of impulses of FIGURES '2c and 2d is again independent of the frequency of the control voltage, but must however lie above such. Advantageously, this pulse repetition rate or pulse recurrencek frequency is adjustable, as will be explained hereinafter.

The mode of operation and the construction of the individual power circuits and control circuits will now more fully be discussed hereinafter with reference to FIGURE 3, which illustrates a circuit diagram of the frequency transformer or converter. In the drawings the conductors which transmit the power have generally been shown in thickened black lines. In the illustrated embodiment the power output stage B with the ignition circuit D is depicted as parallel inverter. However, it is equally pos- This provides the same result.

As has already been mentioned with respect to FIG- URE 1, the illustrated frequency transformer or converter is so constructed that it can be connected toa commercial power supply of, for example, 220 volts and operating at a frequency of 50 c.p.s. In order to connect such with the power supply there is provided a transformer 10 which, in addition to the primary winding 11 is provided with three secondary windings 12, 13 and 14. The secondary winding 12 is operatively connected to the power rectification stage A comprising the semiconductor rectifier elements 35, 36, 37 and 38 arranged in a full-Wave bridge rectification circuit. vThe rectifier elements 35 and 36 can be standard barrier layer rectifiers, whereas as previously mentioned the rectifier elements 37 and 38 are controlled rectifier elements with pnpn-semiconductor barrier layers.

The two controlled rectifiers 37 and 38 which are electrically coupled to the conductor 16, which can be viewed as the positive pole yor terminal of the direct-current voltage source, are ignited iby the ignition circuit C by means ofthe aforementioned series of impulses, in such a imanner that the full-wave bridge rectification circuit A operates as a normal bridge circuit. If the ignition impulses from the control ignition circuit C do not appear, then a direct-current voltage can no longer be produced, so that the power network connected to the primary winding 11 is no longer loaded by the frequency transformer. Both of the non-controlled rectifier elements 3S and 3 6 are connected to the conductor 15 which represents the negative pole or terminal of the direct-current outlet side of the rectification sta-ge A. Between the two poles or Ytermina-ls of the conductors 1S and 16 there is provided in known manner a smoothing capacitor 31. The conductors 15 and 16 conduct the direct-current voltage produced in the power rectification stage A to the power output stage B, in the latter of which this direct-current .voltage is transformedinto an alternating-current voltage of predetermined frequency. f

The power output stage B operating as a direct-current to alternating-current inverter contains `an output transformer 42 provided with a primary winding 43 and a secondary or output winding 44. The primary winding 43V possesses a center-tap 45 which divides the primary v winding 43 into the winding sections or portions 43a a choke or impedance coil 47, connected to the respective cathodes of two controlled semiconductor rectifier elements 50 and 51. The `controlled rectifier elements 50 land 51 are in principle similarly constructed as the controlled rectifier elements 37 and 38 of the power rectification stage A. The anodes of the controlled rectifiers 50 and 51 are each lconnected with a respective end-point or terminal of the primary winding 43 of the output transformer 42 ythrough the agency of the conductors 52 and 53, respectively, and the diodes 52a and 53a, respectively. Finally, the controlled rectifiers Sti and 51 are each bridged by a series arrangement or connection comprising a resistor Stb and Slb, respectively, and a diode 50c and 51C, respectively. The anodcs of the diodes 50c and Slc are, for example, electrically coupled via a conductor l'tSa directly to the conductor 15. Further, between or across the conductors 52 and 53 there is connected a capacitor 59 which influences commutation, as such will be more `specifically described hereinafter.

In order to more fully explain the operation of the power output stage B, it is to be assumed that for -a given period of time the controlled rectier 51 is ignited, so that a current can flow in the direction of the arrow 51. The current thereby flows Ifrom the positive pole or terminal `of the .rectification stage A, that is through the conductor 16 lover the transformer Winding section 43h, the diode 53a, the conductor 53, the controlled rectifier 5l, the conductor 48, the choke coil 47 and the conductor to the negative outlet pole or terminal of the power rectifier stage A. The current thereby flows through the current circuit designated with reference numeral Il, `where in consequence thereof the change of flux associated with the rise in current induces a voltage in the secondary 44 of the transformer 42. At the same time, there is induced a voltage in the other winding section of the primary winding 43, that is in the winding section 43a, so that there is formed at the terminals of the entire primary Winding 43 a voltage with the polarity indicated in parentheses by and in a manner shown in FIG- y-URl-E 3 of the drawing. Correspondingly, the capacitor 59 charges. It will thus be recognized that a voltage is applied during this period of time to the blocked controlled rectifier 5t) in the -blocking or high resistance direction, which voltage corresponds to double the output vvoltage of the power rectifier circuit A. The controlled rectifiers 50 and 51 must thus be dirnensioned in such a manner that they are able to withstand this double voltage.

It is now to bev assumed that the controlled rectifier 50 is ignited, so that its resistance in the forward direction becomes negligibly small. Thus, there is applied to the controlled rectier 5l for a short period of time the stored charge of the capacitor 59, and indeed, such is applied in the blocking or backward direction so that this controlled rectifier element 511 is extinguished. Since the controlled rectifier element Sil is now ignited a current flows in the current circuit l, which comprises the conductor 16, the primary Winding section 43a of the output transformer 42, the diode 52a, the conductor 52, the controlled rectifier 5d, the conductor 43, the choke coil 47 `and the conductor 15. It will thus be seen that, in the second phase of the alternating-current voltage at the output side of the frequency transformer `the polarity at the winding 43 is reversed. The capacitor 59 charges, in contrast with the first phase, with opposite polarity and thereby stores the charge which is necessary after a renewed ignition of the controlled rectifier 51 to bring about an extinguishing of the controlled rectifier 50. The core of the transformer 4Z during this phase is magnetized in the opposite direction, so that on the secondary side 44 of this transformer an alternatingcurrent voltage can be removed.

The diodes Stic, 5 1c are provided in order to achieve 4that the commutation capacitor 59 after commutation .can quickly discharge as well as also quickly charge in the opposite direction. The diodes 52a and 53a, on the other hand, ensure that the capacitor 59 is `able to retain as high a charge as possible and will not partially discharge during a dropping of the voltage at the primary winding 43 of the Itransformer 42 during a half-wave of the output voltage, for example as a result of a pronounced load appearing at the output.

There exists the danger particularly during idle operation, that is without a load at the secondary 44 of the transformer 42, that the voltage amplifies. The capacitor S9 and the primary winding 43 of the transformer 42 thereby operate as an oscillating circuit. In order to prevent such voltage amplitude increases there is arranged parallel to both of the primary winding portions 43a and 4311 a respective resistor 55 and 56. Shortly after the lirst phase, that is when the polarity of the voltage at the transformer `42 corresponds to the signs indicated in parentheses in FIGURE 3, the capacitor 59 can discharge through the resistors 55 .and S6. The rating or size of the resistors 55 and 56 must thus be such that the charge can still be maintained until, for example, the controlled rectifier S0 is extinguished. The resistors 55 and 56 thereby dampen the oscillating circuit, so that the voltage cannot increase in amplitude. It is further to be mentioned that the capacitor 59 can, of course, also be coupled to the secondary winding 44 of the transformer 42, whereby the same mode of operation is achieved.

There will now hereinafter be described the ignition circuits C and D for the controlled rectifier elements. In order to generate the feed or supply voltage for the ignition yor control circuit C there is connected to the secondary Win-ding 13 of the input transformer 10, a full-Wave rectifier circuit 60 which contains `four individual rectifier members. In order to smooth the directcurrent voltage produced in this manner, there is provided a smoothing capaci-tor 64. The negative pole of the direct-current voltage derived in this manner is formed by the conductor 65, and is coupled with both of the connected cathodes of the power rectifiers 37 and 3S via the conductor 20.

The control or ignition circuit C contains a so-called double-base transistor 80, the two bases of which are connected with the negative and positive poles or terminals of the supply voltage source via the resistors 8,1 and `8'2, respectively, that is with the conductors 65 and 72 respectively. Such double-base transistors, or unijunction transistor-transistors of such type, are available on the market. They have the characteristic of suddenly or unsteadily changing the resistance between a base electrode and the emitter as a function of the Noltage at such emitter. If, in the previously described embodiment, the potential at the emitter lies below a predetermined critical value, then the resistance between the emitter and the one base electrode is high. If the voltage at the emitter increases, for example by the charging `of a condenser to this critical value, then the point will be reached at which the resistance between the base electrode and the emitter will suddenly become smaller. The emitter in the above-described instance is connected with the conductor via a capacitor 83- and is connected to the conductor 72 via a resistor 84. The capacitor AS3 and the resistor S4 thereby form an RC- member. `The capacitor 83 charges up to the critical voltage in accordance with the time-constant of the RC- member, whereupon at the resistor 81 a positive impulse appears in consequence of the discharging of the capacitor 83 through the transistor 80. After the discharge of the capacitor 83 the voltage Ifalls below a predetermined value and the transistor again becomes blocked, so that the charging of the capacitor can begin again. The system formed of the members Siiet thereby operates as an oscillator o-r impulse generator, whereby the pulse recurrence frequency is a function of the dimensions of the RC-member. In the aforementioned case, the frequency is considerably higher than that of the transistor.

,transformer 10 a rectification bridge circuit 61.

power supply network, so that for each half-.wave a larger number -of impulses arrive at the controlled rectiers 37 and 38. The pulse repetition frequency of the ignition impulses of ignition circuit C is larger than the frequency of the input Ialternating-current voltage to be rectified, and preferably exceeds such by at least a factor of 3. The rectiers A37, 38 are thereby also ignited during each half-wave if, for any reason, such should become extinguished. Y The impulses appearing at the resistor 81 are transmitted to the ignition electrodes of the controlled rectiiiers 37 and'38 through the intermediary of the conductor 19 Vand via a respective protective resistor Z1 and 22, as best seen by an inspection of the circuit diagram of FIGURE 3. The two controlled rectifiers 37 and 38 are thereby always then ignited when the resistance between the two base electrodes and the emitter of the transistor 80 suddenly changes and when a voltage appears in the forward direction at the controlled rectiiiers. This is always the case with one of the two controlled rectiiiers. Arranged in parallel to the capacitor 83 is a further transistor 89 which short-circuits the capacitor 83 when no negative potential or voltage appears at its base electrode. Between the base of the transistor 89 and the conductor 65 there is disposed the direct-current side of a full-wave rectification circuit 90, the alternating-current side of which is coupled to the secondary winding 26 of the input transformer 24. As has been previously mentioned, the control alternatingcurrent voltage is delivered to the primary side of the transformer 24. Arranged in parallel to the direct-current side of the bridge rectification circuit 90 is a capacitor 91 and a protective diode 92.

The base electrode of the transistor 89 is further connected via a resistor 93 to the conductor 72 which repre- -sents the positive pole or terminal of the power rectifier 60. lf, now, a voltage does not appear at the secondary winding 26 of the transformer 24, then the base of the transistor 89 is held at a positive potential through the resistor 93, so that the transistor S9 is conductive and the capacitor 83 cannot charge. A consequence hereof is that, the transistor 80 is blocked and no ignition impulses can be delivered to the controlled rectiiiers 37 and 38. Thus, a direct-current voltage does not appear between the conductors 15 and 16. ,On the other hand, if there appears a signal at the terminals 23 of the transformer 24 then the capacitor 91 charges so that there appears a negative potential at the base of the transistor 89 which blocks this The result of this is that, the capacitor S3 can charge `and ignition impulses can again arrive at the controlled rectifiers 37 and 38. lf the control alternatingcurrent voltage does not appear at the terminals 23, the capacitor 91 discharges through the protective diode 92 soV that the transistor S9 a gain becomes conductive and impulses no longer appear at the transistor or resistor 81.

The ignition circuit D serves to produce the control impulses for the controlled rectiiiers 50 and 51 in the power output state B designed as a direct-current to alternating-current inverter. In order to produce the supply direct-current voltage for the ignition circuit D, there is electrically coupled to the secondary winding 14 'of the The positive pole or terminal of the thus constructed directvcurrent voltage source is formed through the conductor 62 and the negative pole by the conductor 63 or 63. In order to smooth the direct-current voltage there is connected across these conductors 62 and 63 a smoothing ycapacitor 95. In order to generate the impulses for the controlled rectiiiers 50 and 51 there is here provided two individual impulse generators which are identical to one another and are essentially similarly constructed as the impulse generator for the controlled rectiiers 37 and 38.

pearing inthe upper-half of the illustrated ignition circuit D shown in FIGURE 3, and denoted by reference numerals without a prime marking.

The impulse generator for the controlled rectifier again contains a double-base transistor 100 which is connected with the conductors 62 and 63 through the resistors 101 and 102. The emitter is likewise connected with these conductors 62 and 63 through the intermediary Iof an adjustable resistor 103 and a capacitor 104. The

- pulse repetition rate or pulse recurrence frequency adjustable by means of the resistor 103, is transmitted to the control or ignition electrode of the controlled rrectifier 50 via the conductor 30 and a protective resistor 106. The frequency determining elements 103 and 104 are so dimensioned or adjusted that, the frequency is considerably larger than the frequency of the voltage which is delivered from the power output stage B, or the frequency of the control voltage.

A transistor 107 is arranged in parallel to the capacitor 104 which in the absence of a negative control voltage at its base electrode provides a very small resistance, that is the oscillating system is blocked by short-circuiting of the capacitor 104. The base electrode of the transistor 107 is connected with the conductor 62 via a resistor 108 and with the conductor 63 via a diode 105. Further,

.the base electrode is coupled to one terminal of the secondary winding 27 of the transformer 24, the primary winding of which is fed with the control voltage whose frequency is to be delivered from the frequency converter. The other terminal -of the secondary winding 27 is connected with the base electrode of the corresponding transistor of the second impulse generator, in a manner shown in FIGURE 3. This transistor of the second impulse generator is designated by reference numeral 107. `It will now become apparent that the two transistors 107 and 107 are alternatingly blocked and opened whereby such alternation always occurs during the zero crossover of the control alternating-current voltage. Correspondingly, always only one impulse generator can deliver the necessary series of impulses for igniting the associated controlled rectifier. For example, during the upper halfwave of the control alternating-current voltage the upper impulse generator of ignition circuit D and shown in FIGURE 3, delivers a series of impulses so that the controlled rectifier 50 is ignited, whereas in the second lower half-wave the lower arranged impulse generator of this FIGURE 3, delivers a series of impulses to the rectifier 51 for its ignition, in the same ymanner asV such has been described in the previous discussion of FIGURES 2c and 2d. It will be recognized that the other vimpulse generator of ignition circuit D is similar to the one just described, and has corresponding elements identified with like reference numerals but further bear prime markings.

It is here to Vbe mentioned that the controlling of control voltage does not necessarily haveV to be sinusoidal or regular. On the contrary, it is possible to deliver via the input transformer 24 a series of impulses in which the positive and the negative half-waves appear for different lengths of time. Then also the output voltage at the secondary winding will be correspondingly formed. This can then -be of particular importance when the frequency converter is employed as a generator for spark erosion processes. Since the frequency converter immediately begins to operate as soon as a signal appears, that is also within the contines ofv a half-wave ofV the control voltage, such converter is exceptionally suitable for power command computers in electrical networks. All of the delivered-in or supplied series of impulses appear practically inertialess at the output. As already mentioned, practically no load is removed from the supply power network when an input signal does not appear at the input transformer 24.

The safety circuit E serves the purpose of preventing destruction or damage of the rectiiiers 50 and Sl'if, for example, the output side is short-circuited or when a controlled rectifier is not extinguished in the manner as desired in consequence of a capacitive load. As previously mentioned, the safety circuit E operates as a logical element or circuit. For determining the input signal there is provided the conductor 110, one end of which is connected with the negative pole or terminal of the power rectification stage A or with the conductor 15, in a manner as shown. For determining the output signal twodiodes 111 or 112 are coupled to the conductors 52 and 53, the junction point of which is connected to the conductor 120. The conductor 110 is connected to the conductor 120 via two resistors 114 and 119. The conductor 120 is connected via a diode 116 with the emitter of the transistor 80 and via a resistor 115 with the conductor 72. Furthermore, there is arranged between the conductor 72 and the junction point of the resistors 114 and 119 a capacitor 117.

There are now three operating conditions possible. The first operating condition is that occurring by switching-in, that is at that period of time during which there does not appear a voltage at the conductors 15 or 110, or at the conductor 120. In this instance, the ignition circuit C operates in a normal manner since no voltage whatsoever effects the base electrode of the transistor 80, which could disadvantageously influence its mode of operation. The voltage will adjust itself in accordance with the dimensioning of the capacitor 83 and the resistor 84, in a manner as described hereinabove. In the second condition of operation the normal operating voltage appears at the conductor 110 as well as at the conductor 120. The voltage at the conductor 120 is thereby maintained at a positive potential, so that likewise no potential can reach the emitter of the transistor 80 which could disadvantageously infiuence such.

If, on the other hand, in accordance with the third conditionof operation, there appears a voltage at the conductor 110 and there is missing a voltage at the conductor 120, for example in view of a short-circuit, then in consequence of the negative potential which is transmitted lto the conductor 120 via the conductor 15, the diode 116 is conductive and the base of the transmitter 80 receives a negative bias voltage, so that the system cannot oscillate. The system then remains blocked for such length of time until the capacitor 117, which s arranged in parallel with the resistors 115, 119 is discharged. In the event that the short-circuit or the loading for a short period of time which has caused the disturbance has in the meantime been corrected, then the system after a lrenewed build-up or oscillation of the ignition circuit C will again work in the normal desired manner. The safety circuit E is particularly meaningful because of the fact that one of the controlled rectifiers 50 or 51 in consequence of special load requirements cannot be extinguished, so that both controlled rectifiers f) and 51 are ignited at the same time. The dimensioning of the capacitor 117 and the resistors 115, 119 must be so f selected that the transistor 80 becomes blocked for such length of time until the two controlled rectifiers 50 and 51 are positively extinguished. If such is the case, then the frequency converter will continue to operate, whereby the output voltage in consequence of the fact that the ignition results by a series of impulses, will immediately adjust itself to the condition which is determined by the alternating-current voltage appearing at the terminals 23 of the input transformer 24.

Quite obviously, the inventive frequency transformer is capable .of additional uses as will be apparent to those versed in the art. Thus, for example, the inventive frequency transformer can also be employed as generator for inductive heating, for generation of ultrasonics, for illuminating purposes, and for welding techniques. Further, such frequency transformer can be employed for command installations for power supply networks.

Having thus described the present invention, what is desired to be secured by United States Letters Patent is:

1. A frequency transformer comprising a power rectification stage and a power output stage for converting the generated direct-current voltage of the power rectification stage into an alternating-current voltage of predetermined frequency, a source of power for said power rectification stage, said power rectification stage and said power output stage each being provided with controlled semi-conductor rectifiers, first and second ignition circuits, said first ignition circuit delivering ignition impulses for said controlled semi-conduction rectifiers of said power rectification stage, said second ignition circuit delivering ignition impulses for the cont-rolled semi-conductor rectifiers of said power output stage, safety circuit means constructed as a logical circuit, said safety circuit means having its input coupled to the input and output of said power output stage and having its output coupled with said first ignition circuit, such that said logical circuit prevents the transmission of said ignition impulses to said controlled semi-conductor rectifiers of said power rectification stage when the voltage appearing at the power loutput stage is at least a smaller predetermined value than the voltage appearing at the input thereof.

2. A frequency transformer according to claim 1; wherein said first ignition circuit for said ignition impulses for said controlled semi-conductor rectifiers of said power rectification stage includes an impulse generator which can be blocked by an output signal of said safety circuit means.

3. A frequency transformer according to claim 1; wherein said first ignition circuit for igniting said controlled semi-conductor rectifiers of said power rectification stage includes means for delivering a series of impulses possessing a pulse recurrence frequency which is greater than the frequency of the alternating-current voltage which is to be rectified.

4. A frequency transformer according to claim 3; wherein said pulse recurrence frequency exceeds lsaid frequency of said alternating-current voltage to be rectified by a factor of 3.

5. A frequency transformer according to claim 1; wherein said second ignition -circuit comprises a pair of controlled semi-conductor rectifiers, said second ignition circuit alternatingly transmits a series of impulses to one of both of said controlled semi-conductor rectifiers within each half-wave of the output alternating-current voltage.

6. A frequency transformer according to claim S; wherein said second ignition circuit during each half-wave of said output alternating-current voltage transmits to one of said pair of controlled semi-conductor rectifier means at least three individual impulses.

7. A frequency transformer according to claim l; including an input transforme-r for delivering a control voltage for controlling the frequency of the output alternating-current voltage, said input transformer being provided with a secondary winding for controlling said second ignition circuit for igniting said controlled rectifiers of said power output stage.

8. A frequency transformer according to claim 7; wherein said input transformer for said control voltage is provided with a further secondary winding electrically coupled with said first ignition circuit for igniting said controlled rectifiers of said power rectification stage 1n such a manner that said first ignition circuit only then transmits ignition impulses when the voltage at the input transformer for the control voltage exceeds a predetermined value.

9. A frequency transformer according to claim 1; wherein said safety circuit means includes a time delay component which ensures that after blocking of said first ignition circuit the latter only after lapse of a predetermined period of time again begins to deliver ignition impulses for said power rectification stage.

10. A frequency transformer according to claim 1;

l l Q3 wherein the output side of the `frequency transformer is OTHER REFERENCES immuable to a Workgap of an electro'emsion machine- General Electric Controlled Rectifier Manual (March 1960), published by Semiconductor Products Department References Cited by the Exfmm of GE., FIGURE 7.16, pp. 101-103 and FIGURE $.29,

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P y lo Y ASSS-RVEJCQITLHZI'S.

1,303,427 7/1962 France. 

1. A FREQUENCY TRANSFORMER COMPRISING A POWER RECTIFICATION STAGE AND A POWER OUTPUT STAGE FOR CONVERTING THE GENERATED DIRECT-CURRENT VOLTAGE OF THE POWER RECTIFICATION STAGE INTO AN ALTERNATING-CURRENT VOLTAGE OF PREDETERMINED FREQUENCY, A SOURCE OF POWER FOR SAID POWER RECTIFICATION STAGE, SAID POWER RECTIFICATION STAGE AND SAID POWER OUTPUT STAGE EACH BEING PROVIDED WITH CONTROLLED SEMI-CONDUCTOR RECTIFIERS, FIRST AND SECOND IGNITION CIRCUITS, SAID FIRST IGNITION CIRCUIT DELIVERING IGNITION IMPULSES FOR SAID CONTROLLED SEMI-CONDUCTION RECTIFIERS OF SAID POWER RECTIFICATION STAGE, SAID SECOND IGNITION CIRCUIT DELIVERING IGNITION IMPULSES FOR THE CONTROLLED SEMI-CONDUCTOR RECTIFIERS OF SAID POWER OUTPUT STAGE, SAFETY CIRCUIT MEANS CONSTRUCTED AS A LOGICAL CIRCUIT, SAID SAFETY CIRCUIT MEANS HAVING ITS INPUT COUPLED TO THE INPUT AND OUTPUT OF SAID POWER OUTPUT STAGE AND HAVING ITS OUTPUT COUPLED WITH SAID FIRST IGNITION CIRCUIT, SUCH THAT SAID LOGICAL CIRCUIT PREVENTS THE TRANSMISSION OF SAID IGNITION IMPULSES TO SAID CONTROLLED SEMI-CONDUCTOR RECTIFIERS OF SAID POWER RECTIFICATION STAGE WHEN THE VOLTAGE APPEARING AT THE POWER OUTPUT STAGE IS AT LEAST A SMALLER PREDETERMINED VALUE THAN THE VOLTAGE APPEARING AT THE INPUT THEREOF. 